1. Field of the Invention
The invention concerns radiology instruments and, more particularly, in such instruments, a safety device for the radiogenic unit comprising the X-ray source (the tube) and the means providing protection against ionizing rays and electric shocks.
2. Description of the prior Art
X-ray tubes, for medical diagnosis for example, are generally set up like a diode, namely with a cathode and an anode or anti-cathode, these two electrodes being enclosed in a vacuum-sealed envelope that enables electrical insulation to be set up between these two electrodes. The cathode produces a beam of electrons and the anode receives these electrons on a small surface which constitutes a focus or target from which the X-rays are emitted.
When the high supply voltage is applied to the terminals of the cathode and the anode, so that the cathode is at the negative potential, a current known as an anode current is set up in the circuit, through a generator producing the high supply voltage. The anode current flows through the space between the cathode and the anode in the form of a beam of electrons which impinge on the target.
A small proportion of the energy dissipated to produce the electron beam is converted into X-rays. The rest of this energy is converted into heat. Thus, in view also of the high instantaneous power values (in the range of 100 kw) brought into play and the small dimensions in the range of one millimeter) of the target, manufacturers have long been making rotating-anode X-ray tubes where the anode is made to rotate to distribute the thermal flux on a ring called the focal ring, having a far greater area than the focus, the usefulness thereof being all the greater as the rotational speed is high (generally between 3,000 and 12,000 rpm).
The standard type of rotating anode has the general shape of a disk with an axis of symmetry around which it is made to rotate by means of an electric motor. The electric motor has a stator located outside the envelope of the X-ray tube and a rotor which is mounted within this envelope and positioned along the axis of symmetry. The rotor is mechanically fixed to the anode by means of a supporting shaft.
The energy dissipated in a tube of this kind is high and there is therefore provision for cooling it. To this end, the tube is enclosed in a casing wherein a cooling fluid, notably oil, is made to circulate. The fluid itself is cooled in a heat-exchanger which may be of the air or water type. Thus, a cooling device that works permanently has been made. However, the X-ray tube emits only intermittently so that the dissipated energy is substantial during the examination stage itself, which lasts some from a few seconds to a few some minutes, and it is practically null for the time during which no patient is examined. The result thereof is major disparities in the quantity of heat to be removed, depending on the phase considered. This leads to major variations in the temperatures of the materials used for the tube. These variations may hamper the proper working of the tube.
The oil contained in the casing is thus subjected to increases in temperature which have the effect of an expansion in the volume of oil and, consequently, an excess pressure within the casing. In order take this expansion into account in the normal range of operation of the tube, the casing is fitted out with a membrane which, when moving, increases or reduces the volume of the casing containing the cooling oil.
However, there may be increases in temperature and, hence, degrees of expansion that exceed those for which the expansion membrane is designed. The result of this is excess pressures which may damage the casing (for example by the tearing of the expansion membrane) or the tube (for example, by causing it to explode). Accidents such as this, apart from putting the radiology equipment out of working order, are a danger to patients and users.
Thus, to prevent such accidents, the casings are fitted out with alarm devices that detect any excessive increase in the volume of the casing, namely a shifting of the expansion membrane, and give an alarm signal, for example by means of a microswitch associated with said membrane. Other alarm devices measure the temperature or the pressure and give an alarm signal when the measured values go beyond a certain threshold. These different alarm devices, which are triggered by a variation of expansion, temperature or pressure, are often used simultaneously to reinforce the probability of detection of an abnormal working condition, and the first alarm signal that appears usually switches off the high-voltage generator for it is the main source of heat.
Despite these devices, accidents may occur for the following reasons. Firstly, all the alarm devices may be malfunctioning or out of order but this is an extremely rare possibility. Then, it may be that the generator switch device is malfunctioning and has not worked despite the alarm signal, so that the high voltage remains applied to the tube. Such a case is also very rare. Finally, a less infrequent case is the one where users neutralize the safety systems installed by the manufacturers because they feel that the triggering thresholds are too low to enable them to carry out all the series of examinations that they need.
It is an object of the present invention, therefore, to set up a safety system that acts independently of the cut-off device of the high voltage generator, thus eliminating the risks that result from the malfunctioning of the cut-off device.
Another object of the present invention is to make a safety device that cannot be neutralized by users.